The possibility of identifying user geographical location in the network has enabled a large variety of commercial and non-commercial services, e.g., navigation assistance, social networking, location-aware advertising, emergency calls, etc. Different services may have different positioning accuracy requirements imposed by the application. In addition, some regulatory requirements on the positioning accuracy for basic emergency services exist in some countries. Emergency 911 services in the U.S. (FCC E911) stands as one example of regulation-driven requirements.
In many environments, the position can be accurately estimated by using positioning methods based on GPS (Global Positioning System). Nowadays networks also often have a possibility to assist items of user equipment (UEs), to improve their receiver sensitivity and GPS startup performance (referred to as Assisted-GPS positioning, or A-GPS). GPS or A-GPS receivers, however, may not necessarily be available in all wireless terminals. Furthermore, GPS is known to often fail in indoor environments and urban canyons. A complementary terrestrial positioning method, called Observed Time Difference of Arrival (OTDOA), has therefore been standardized by 3GPP.
With OTDOA, a terminal measures the timing differences for downlink reference signals received from multiple distinct locations. As an example, a particular UE receives downlink reference signals from a supporting or reference cell, and from a number of neighboring cells. For each (measured) neighbor cell, the UE measures a Reference Signal Time Difference (RSTD) which is the relative timing difference between a neighbor cell and the reference cell. The UE position estimate is then found as the intersection of hyperbolas corresponding to the measured RSTDs. At least three measurements from geographically dispersed base stations with a good geometry are needed to solve for two coordinates of the terminal and the receiver clock bias of the terminal. Positioning calculations can be conducted, for example, by a positioning server (E-SMLC or SLP in LTE) or UE. The former approach corresponds to the UE-assisted positioning mode, whilst the latter corresponds to the UE-based positioning mode.
To enable positioning in LTE and facilitate positioning measurements of a proper quality and for a sufficient number of distinct locations, new physical signals dedicated for positioning (positioning reference signals, or PRS) have been introduced and low-interference positioning subframes have been specified in 3GPP. See 3GPP TS 36.211, Evolved Universal Terrestrial Radio Access (E-UTRA); Physical Channels and Modulation, for more detailed information on PRSs.
Broadly, PRSs are transmitted according to a predefined pattern and following one of the predefined PRS configurations, each defined by: a PRS transmission bandwidth, the number of consecutive positioning subframes (NPRS) defined as a PRS positioning occasion, and a PRS occasion periodicity of TPRS, measured in subframes, i.e., the time interval between two positioning occasions. FIG. 1 depicts this definitional arrangement for subframe allocation in a given network cell. (Note that a “cell” refers to a defined coverage area, e.g., under the control of a given base station. Each base station within the radio access portion of the network may control one cell, or more than one cell, but reference signals generally are transmitted for each such distinct cell.) The values currently allowed by the standard of TPRS are 160, 320, 640, and 1280 subframes, and the number NPRS of consecutive subframes are 1, 2, 4, and 6 (again, see 3GPP TS 36.211).
Because OTDOA positioning requires measuring PRS signals from multiple distinct locations, the UE receiver must be able to handle the case where some of the PRSs are received at much weaker signal levels. For example, the PRS from a given neighboring cell may be much weaker at the UE than those from the serving cell. As a further complication, a UE without approximate knowledge of when the PRSs are expected to arrive and according to what pattern is obligated to perform signal searching within a large window. Such processing affects the time and accuracy of the measurements, and undesirably increases UE complexity.
Therefore, to facilitate PRS measurements by UEs, the network transmits “assistance data.” Among other things, the assistance data includes reference cell information, neighbor cell lists containing PC's (Physical Cell IDs) of neighbor cells, the number of consecutive downlink subframes occupied by PRSs, PRS transmission bandwidth, frequency, etc.
However, as another complication related to PRS measurement, the PRS transmitted by any given cell can be transmitted with zero or very low power, both of which may be referred to as muting. Muting applies to all PRS resource elements within a certain time period (e.g., one subframe or one PRS positioning occasion) over the entire PRS transmission bandwidth. PRS muting provides a mechanism to reduce interference in PRS measurements, e.g., muting PRS transmission in one cell allows UEs to make better measurements on the PRS transmitted in another cell. While standardized approaches to PRS signaling may exist, no such standardization exists with regard to particular muting patterns used.
Certain approaches to muting have been discussed in the context of 3GPP. One approach relies on random muting by cells, where each base station (eNodeB in LTE) decides whether its PRS transmissions are muted or not for a given positioning occasion according to some probability. In a simple or random implementation, there is no coordination among eNodeBs and the probability is statically configured per eNodeB or per cell. Random muting offers the advantage that no signaling is needed, as each eNodeB makes muting decisions autonomously, according to the configured probability. However, the approach has disadvantages.
For example, real-world networks are not homogeneous. They have different cell coverage areas and user densities, and possibly different types of base stations. These variations imply that setting optimal muting probabilities is a tedious task. Further, random muting does not provide UEs with information on whether a cell is or is not muted for a given positioning occasion, which complicates RSTD measurements and increases the UE complexity. Still further, the optimal configuration of the muting probabilities may also vary, for example, over the day and over the week and on the cell basis, which makes static configurations not the best option from a practical point of view.
Another approach provides a limited set of muting patterns and maps those patterns to PCTs. See, for example, the proposals provided as R1-093793, Muting for LTE Rel-9 OTDOA Positioning, 3GPP TSG-RAN WG1 meeting #58bis, October 2009, and as R1-092628, On serving cell muting for OTDOA measurements, 3GPP TSG-RAN WG1 meeting #57, June 2009.
One advantage of the above mapping-based approach is that given a table of muting patterns and PC's received in the assistance information, any given UE can determine when the PRSs are muted in a given cell of interest without the muting information being explicitly signaled to the UE. However, as a disadvantage, the muting patterns need to be either hard coded in UEs (which implies the solution is not suitable for all UEs) or received from the network for which new signaling would be required.
As a further complication, mapping muting patterns to PC's will most likely not result in an optimal muting configuration in non-uniform real networks that may also have a multi-layer structure. In other words, such a mapping-based muting configuration would be fixed and thus impossible to re-optimize unless PCI planning is redesigned for the entire network specifically for positioning, which is most likely to be the least desired activity from the network operator's point of view.
Other proposals involve the transmission of muting indicators to UEs, indicating whether or not autonomous muting for a given cell is activated. See, e.g., 3GPP RP-100190, Autonomous muting in DL OTDOA, Motorola, March 2010, and see CR to 3GPP TS 36.355, Autonomous muting indication in OTDOA assistance information, Motorola, March 2010. According to such approaches, a Boolean indicator is transmitted for the reference cell and also all neighbor cells, as a part of the assistance data whenever PRSs are transmitted. When the indicator is FALSE, UEs can avoid blind detection of PRS muting, optimize detection thresholds and thus improve the positioning performance. With the indicator set to TRUE, the UE still does not receive the information on when and in which resource blocks (RBs) muting occurs, which means that the UE still needs to blindly detect when PRS muting is used in each cell, i.e. the proposal does not solve the problems associated with blind detection.
As an alternative that simplifies UE requirements, it has been proposed to remove autonomous muting functionality from the LTE Rel. 9 specification. However, such a proposal leaves unaddressed those scenarios which have been shown to require muting.